How do I solve "Color management: using fallback mode for management"
This is normal but you can make this message go away by building with OpenColorIO. Since it is the default now for color management in Blender, by not using it in your case here, Blender is falling back to an simpler set of color management rules, where float buffers are linearized sRGB buffers, and byte buffers are sRGB nonlinear buffers.

Note that imports after the first will use the cached module so they are not loading the entire Blender state every time. But this means you can't for eg, have 2+ blend files loaded from Python at once. each Python process can only operate on one blend file at a time.